1. Field of the Invention
The present invention generally relates to synchronization recovery circuits and radiocommunication apparatuses using such circuits, and more particularly, to a synchronization recovery circuit which detects a word synchronizing character in a received data signal for recovering word synchronization in a cellular-type communication system, and to a radiocommunication apparatus such as car telephone and portable telephone using such a synchronization recovery circuit.
2. Description of the Prior Art
A cellular-type communication system has been widely utilized in the form of a mobile radio telephone such as car telephone or portable telephone In such a cellular-type communication system, an area to be covered is divided into multiple cells, each of which is provided with a radio base station and assigned a group of channels. Such a small-sized service area of each cell and reduced transmission output of a base station enable frequent reuse of the frequencies, admitting an increased number of subscribers.
In such a cellular-type communication system, when a mobile station equipped with a radiocommunication apparatus, for example, an automobile equipped with a car telephone moves from one cell to another, several kinds of digital control including channel change or the like are performed in order that complete communications are established between the radio base stations involved and the car telephone apparatus.
Each radio base station has two types of radio channels, one of which is a bidirectional control channel for such several kinds of digital control as mentioned above, and the other is a bidirectional audio channel for conversation over the telephone.
For such bidirectional communications, four channel types are employed. More particularly, the control channel comprises a forward control channel (FOCC) used for communication from a radio base station to a mobile station and a reverse control channel (RECC) used for communication from a mobile station to a radio base station, which are generally used for control of every kind before the audio channel is established, and not for conversation. On the other hand, the audio channel comprises a forward audio channel (FVC) used for communication from a radio base station to a mobile station and a reverse audio channel (RVC) used for communication from a mobile station to a radio base station.
On these channels, a message, or a data signal containing a word synchronizing character is transmitted at predetermined word and bit rates. Therefore, a radiocommunication apparatus of a mobile station is first required to detect the word synchronizing character out of a received data signal for recovering word synchronization. For this purpose, in a conventional radiocommunication apparatus, as disclosed in the U.S. Pat. No. 4,029,900, there is provided a synchronization recovery circuit which detects the word synchronizing character out of the data signal for recovering word synchronization. In this synchronization recovery circuit, the word synchronization is recovered by supplying a control circuit for data processing with a word synchronizing signal which has been generated based on the detected word synchronizing character.
The conventional synchronization recovery circuit, however, is constituted in the light of its hardware construction such that the word synchronizing signal is serially applied to the control circuit. Therefore, in order to recover word synchronization based on such a serial word synchronizing signal, also the data signal must be serially applied to the control circuit. The control circuit receives both of the word synchronizing signal and the data signal serially and accepts the data signal as effective data for use, only after a word synchronizing position is identified. In the above mentioned construction wherein the data signal is adapted to be accepted after word synchronization is established, data may possibly drop out due to high data transmitting rate of the celluar-type communication system. Therefore, the control circuit is always required to access the data signal and thus can not do any other processing such as key scan, LCD display, AF (Analog Frequency) processing, radio-functional processing or the like during the access, prevented from performing any kind of processing required as a mobile station, at a higher speed.
As one solution to this problem, a control circuit comprised of two microprocessors has been proposed, with one microprocessor performing synchronization recovery of the received data and the other performing any other processing such as key scan. This has led to another problem, however, that the two microprocessors require additional complicated control such as communication control (for example, determination of communication protocol) and timing control between themselves.
Furthermore, a technology of converting the data signal from serial form to parallel form before supplied to the control circuit has been proposed in Japanese Patent Laying-Open No. 63-245032. Since the word synchronizing signal is supplied in serial form, however, such a parallel data signal in itself does not allow identification of a synchronizing position, so that any software-controlled processing becomes indispensable in the control circuit. More specifically, such software-controlled processing may include a processing for determining a synchronizing character pattern by shifting a data signal bit by bit in order to identifying the position of the synchronizing character in the accepted data signal again. Therefore, with such a technology, a higher operating speed of the radiocommunication apparatus can hardly be attained.